A Venturi valve is a valve device that may be used to regulate, among other things, air flow velocity. In general, a Venturi valve has an input connected to a source of air flow and an output at which regulated air flow is provided. The Venturi valve is self-adjusting, ideally maintaining a constant output air flow velocity irrespective of changes in source air pressure or flow velocity. To this end, the Venturi valve incorporates valve geometry and a spring-loaded mechanism that cooperate to maintain the more or less constant output flow velocity. When source air pressure increases, the pressure against the spring-loaded mechanism urges the valve further closed, thereby compensating for the increase in the source flow velocity. Similarly, when the source flow pressure decreases, the reduced pressure allows the spring-loaded mechanism to urge the valve further open, thereby compensating for the reduction in source flow velocity.
In addition to being self-adjusting to maintain a constant air flow velocity, Venturi valves are preferably adjustable to provide a selectable level of air flow. In other words, a Venturi valve may be adjusted to provide a select output air flow velocity, and then the Venturi valve will self-regulate to maintain that select air flow velocity. To perform the adjustment, an electro-mechanical actuator or other type of actuator adjusts the position of a shaft that holds the stopper or cone of the valve. The position of the shaft defines the output air flow velocity.
Venturi valves have a number of applications, including those in the building control system industry. Venturi valves may be used to control air flow into or out of a room in a building, or to control exhaust air flow from a fume hood in a laboratory environment. By way of example, U.S. Pat. No. 4,215,627 describes the use of a rudimentary Venturi valve in a fume hood environment. U.S. Pat. No. 4,215,627 does not describe, however, the use of an adjustable Venturi valve.
More recently, U.S. Pat. No. 5,304,093 to Sharp et al. (hereinafter the Sharp patent) describes an adjustable Venturi valve that may be used in a laboratory environment. One issue that arises with adjustable Venturi valves is determining the proper shaft or crank arm position that is necessary to achieve a desired amount of flow velocity. To address this problem, the Sharp patent describes a control circuit having an input/output relationship that correlates shaft position to desired flow values. In operation, an input signal representative of a desired flow value is provided to the control circuit, and then the control circuit generates an output signal representative of a corresponding shaft position. The output signal is conditioned and provided to an actuator that positions the shaft accordingly. Closed-loop control may be used to fine tune the shaft position. Once the shaft is in position, the Venturi valve operates to maintain the desired flow regardless of input air flow.
One problem with adjustable Venturi valve described in the Sharp patent is that the relationship between the shaft position and the output air flow velocity necessarily varies from device to device, due to manufacturing tolerances and natural variations in the manufactured springs used in Venturi valves. Thus, each device must ordinarily be calibrated to ensure that the relationship between shaft position and output air flow velocity is reasonably accurate.
Venturi valves are calibrated in the factory using special calibration fixtures. While factory calibration has advantages, it cannot account for changes in the operation of the valve arising from mounting considerations. Moreover, specific environmental conditions to which the Venturi valve is exposed can affect its calibration accuracy. Thus, when the valve is finally installed, the factory calibration may be somewhat inaccurate.
There is a need, therefore, for a way of avoiding or compensating for calibration errors that can result from alteration of operating parameters that occur upon installation of a Venturi valve in its final operation location.